This invention relates to a heat transfer sheet, more particularly to a heat transfer sheet which is useful for a heat transfer system by use of a sublimable dye (heat migratable dye), excellent in dye migratability during heat transfer, and also can give excellent image density.
As the method for giving excellent monocolor or fullcolor image simply and at high speed in place of the impact printing or general printing method, non-impact printing such as on ink jet system or the heat transfer system has been developed. Among these, the so called sublimation heat transfer system by use of a sublimable dye is the most excellent as one having excellent continuous gradation and giving fullcolor image comparable with color photography.
The heat transfer sheet to be used in the sublimation type heat transfer system as mentioned above may be generally one having a dye layer comprising a sublimable dye and a binder formed on one surface of a substrate film such as polyester film, and a heat-resistant layer provided on the other surface of the substrate film for prevention of sticking of a thermal head.
By superposing the dye layer surface of such a heat transfer sheet on an image receiving material having an image receiving layer comprising a polyester resin and the like, and heating imagewise from the back of the heat transfer sheet by a thermal head, the dye in the dye layer is transferred to the image receiving material to form a desired image.
In the heat transfer system as described above, only the dye is migrated from the dye layer to the image receiving material, and the binder remains on the substrate film side. In this case, sharper and higher density images can be formed, as the migratability of the dye is better.
As the method for improving migratability of the dye, it is the simplest to increase printing energy, but higher printing energy undesirably results in increased printing cost. Further, when a plastic film is used as the substrate film, the thermal energy which can be applied is of itself limited.
As another method, it has been well known in the art to use a dye of low molecular weight, but when the molecular weight of the dye is low, there ensues the problem that fastness of the image formed, such as bleed resistance and heat resistance, is inferior.
As the method for circumventing such problems, the method of using a dye with a high molecular weight has been known. However, use of these dyes with high molecular weights involves the problem that migratability of the dye becomes inferior, thus preventing the formation of a sharp and high density image.
Accordingly, a first object of the present invention is to provide a heat transfer sheet which can form an image of satisfactory density with lower printing energy as compared with the prior art, or can form an image of higher density with the same printing energy as in the prior art.
Meanwhile, another known method for improving migratability of the dye is to add a compound having a low melting point such as wax, etc. as the sensitizer into the dye layer. However, addition of these low melting compounds gives rise to problems such as blocking of the heat transfer sheet wound up in a roll, bleeding of the dye to be transferred to the back, etc. Further, during heat transfer, there ensues the problem that the dye layer tends to be fused onto the surface of an image receiving material to be peeled off with difficulty. When it is peeled off, the dye layer tends to be migrated to the image receiving material.
An addition of fine particles such as silica as the release agent into the dye layer may be conceivable for solving these problems. However, in this case, the transferred image becomes coarse, thereby causing the problem that color reproducibility and resolution become low. Also, there is the method of adding a silicone oil as the release agent, but such silicone oil has no compatibility with the dye layer, whereby problems such as generation of surface stickness, occurrence of discoloration of the transferred image, reduced storability, and the like, result.
Therefore, a second object of the present invention is to provide a heat transfer sheet which can form an image of satisfactory density with lower printing energy as compared with the prior art, or can form an image of higher density with the same printing energy as in the prior art without causing problems of storability and fusion to occur.